My simple rules of engagement:

You shouldcontact me before executing the deal/making transactions. This way I can be aware of the turn of events in advanced, making me a better judge.
If you don’t get my approval in advanced this means I can refuse to be the arbitrator when the time comes.

I am not a lawyer, I will be using my common sense & good judgement, and I will not be held responsible for anything. You will not sue me.

I am offering this service free of charge. I will be accepting an optional tip that will have to be of an equal amount from both sides to keep things balanced.